Wireless tag readers are commonly used in industrial plants to identify and track manufactured items. In such applications, a wireless identification tag such as an RFID (Radio Frequency Identification) tag is affixed to a monitored item such as a single packaged product or a bin or pallet containing multiple manufactured items. Typically, the wireless tag stores relevant data pertaining to and identifying the item to which it is affixed. As the wireless tag passes near a wireless tag reader, the tag reader communicates with the wireless tag to retrieve information pertaining to the tag itself and the tagged item.
Some wireless tags are ‘active’ in the sense that they include an antenna and transmitter capable of initiating wireless communication with a tag reader. Some active wireless tags contain only a radio transmitter, while others include a radio receiver. The transmitter may generate its own signal, or it may “backscatter” modulate the reader's transmitted signal. Active wireless tags include a power source such as a battery to power their internal electronic circuitry and generate a wireless communication signal. Typically, because of this on board power source, active tags exhibit longer transmission range (called “read range”) than the passive tags described below.
In comparison to active tags, ‘passive’ wireless tags do not contain a power source and therefore they cannot independently generate a wireless signal to communicate with a tag reader or process digital information on their own. Instead, these passive devices rely on receiving power from an interrogating signal generated by a tag reader to power their internal circuitry. In the presence of the interrogation (or ‘powering’) signal, the passive tag devices are able to power themselves, retrieve data stored in their memory, and communicate the retrieved data to the tag reader. The amount of power that passive tags are able to recover from the reader's signal is generally small, so passive tags generally exhibit shorter transmission range (reduced “read range”) than active tags.
Certain passive tags include a resonant circuit or antenna tuned to a particular interrogation frequency of the tag reader. In such devices, the characteristics of the resonant circuit are altered via switching (e.g., switching a resistor, inductor, or capacitor in and out) to modulate a signal to a tag reader according to a stored data string associated with the tag. A modulated signal produced by switching is then re-radiated to the tag reader. The tag reader, in turn, processes the received data string associated with the tag to identify characteristics of the item or tag itself. In certain applications, the tags can store information received from the tag reader in a memory located on the tag.
In practice, tag readers are typically mounted at strategic locations in manufacturing and/or retail facilities to monitor a presence of wireless identification tags. Mounting tag readers at locations throughout a facility enables tracking movement of wireless tags and, thus, corresponding tagged items. The number of tag readers employed in a facility depends to some extent on the characteristics of the facility, as well as the operating characteristics of the tags (e.g., whether the tag is an active or passive tag, as well as on the tag's operating frequency). Specifically, applicable government radio regulations, tag type, and the size, shape and number of rooms and floors in a building are factors to consider when installing a tag monitoring system.